1. Field of the Invention
The present invention relates to the field of display technology, and in particular to a thin-film-transistor (TFT) substrate manufacturing method and a TFT substrate.
2. The Related Arts
Thin-film transistors (TFTs) are currently the primary driving device of liquid crystal displays (LCDs) and active matrix organic light-emitting diodes (AMOLEDs) and are directly related to the development of high performance flat panel display devices. The TFTs have various structures and there are various materials that are used to make the corresponding ones of the TFT structures. Low temperature poly-silicon (LTPS) material is one of the preferred materials. The regular arrangement of atoms of LTPS makes the mobility of charge carriers high. For the liquid crystal displays that are driven by voltage, due to the relatively high mobility that a poly-silicon thin-film transistor may have, driving liquid crystal molecules to rotate can be achieved with a thin-film transistor having a smaller size. This greatly reduces the space occupied by the thin-film transistor and thus increases the light transmitting area and provides increased brightness and resolution. For AMOLEDs that are driven by current, a LTPS TFT may better suit the need for driving current. Hot carrier effect is an important mechanism of failure of metal oxide semiconductor (MOS) devices and with the size of the MOS devices being increasingly reduced, the hot carrier injection effect becomes increasingly severe. Taking P-type metal oxide semiconductor (PMOS) device as an example, holes existing in the channel are acted upon by an intense lateral electric field established between the drain and source terminals to get accelerated and become high energy carriers. The high energy carriers collide the crystal lattice of silicon and generate electron-hole pairs through ionization. The electrons are collected by a substrate to form a substrate current; and most of the holes generated by the collision flow to the drain terminal, while a fraction of the holes are acted upon by a longitudinal electric field to inject into the gate terminal and form a gate current. This phenomenon is referred to as “hot carrier injection”.
The hot carriers may cause breaking of energy bonds at the silicon substrate and silicon oxide gate oxide interface and generate an interface state between the silicon substrate and the silicon oxide gate oxide interface, leading to deterioration of device performance, such as threshold voltage, transconductance and linear zone/saturation zone currents, and eventually resulting in failure of the MOS device. The failure of the MOS device generally occurs at the drain terminal first. This is because the charge carriers are accelerated by the electric field in the entire channel and when getting to the drain terminal, the energy of the charge carriers reaches the maximum level. Consequently, the hot carrier injection phenomenon is more severe at the drain terminal. Thus, a hot spot of researches of those working in this field would be to alleviate the damage of a semiconductor device caused by hot carrier injection.
For LTPS TFTs, the charge carrier mobility is around 20-100 times of that of amorphous silicon (a-Si) TFTs and they are readily susceptible to hot carrier injection phenomenon. The charge carriers, when moving in an intense electric field (>4E10^4V/cm), may acquire an amount of energy from the electric field that is greater than an amount of energy lost by interaction with the crystal lattice so that the speed of the charge carriers may get higher and higher, eventually resulting in the occurrence of hot carrier injection. To alleviate the damage caused by hot carrier injection, solutions that are commonly adopted are to apply ion injection to form a lightly doped transition zone, such as lightly doped drain (LDD) and gate overlapped lightly doped drain (GOLDD). These solutions, however, are complicated and are readily susceptible to doping deviation to result in failure.